1. Field of the Invention
This invention relates to the field of controlling foamy slag in an electric arc furnace to maximize energy transfer to the slag.
2. Related Art
Foamy slag practices have been used for several years in electric arc furnace (EAF) operations. The slag is produced to cover the arc so that the arc is shielded and energy is transferred to the bath. Oxygen is injected with coal (carbon) to foam the slag by producing CO and CO2 gas in the slag. When foamed, the slag cover increases from 4 to 12 inches. Increase in energy efficiencies have been reported ranging from 60% to 90% with slag foaming, compared to 40% without. Jones, J. “Interactions Between Electric Arc Furnace Operations and Environmental Concerns,” IRON AND STEEL ENGINEER, p 37-45, April 1995. Unfortunately, the benefits of foamy slag are highly dependent on good practices that include estimating and controlling the slag to achieve preferred operating conditions. Operation and material variability make it more difficult to obtain the desired benefits.
The foamy slag is a very complex system in itself. Some researchers have proposed several physicochemical methods to estimate and control the slag. One scientific approach consists on estimating the slag composition based on mass balance calculations and stability diagrams, and changing the slag conditions to attain some basicity properties. Pretorius, E. B., and Carlisle, R. C., “Foamy Slag Fundamentals and Their Practical Application to Electric Furnace Steelmaking,” ELECTRIC FURNACE CONFERENCE PROCEEDINGS, p. 275-291, 1998. This approach requires a broad knowledge of principles in order to achieve proficient slags. Variations in raw materials and practices prevent this method from being adapted to optimum operation.
Other methods use transport phenomena models and empirical correlations to estimate the slag height, Zhu, M., Jones, T., and Sichen, D., “Modeling Study of Slag Foaming by Chemical Reaction,” SCANDINAVIAN JOURNAL OF METALLURGY, Vol. 30, p. 51-56, 2001; Ito, K., and Fruehan, R. J., “Slag Foaming in Electric Furnace Steelmaking,” TRANSACTIONS OF ISS, I&SM, p. 55-60, 1989, but these methods are oversimplified and their results do not cover other variables that can be of importance. Consequently, these methods are not readily adapted for industrial implementation.
The methods found in the literature require taking slag samples and performing off-line analyses, so that after some calculations, the practices can be changed. In other instances, measurements are not available or difficult to implement. More importantly, these methods are not reliable where process variations are encountered that cannot be quantified, such as raw materials quality variations, etc. Therefore, current mill practices are not able to operate EAFs closer to the time varying optimum energy levels as shown in FIG. 1.
Thus, a problem associated with electric arc furnace controls that precede the present invention is that they require difficult and imprecise measurements of slag conditions that are not amenable to reliable measurement.
Still another problem associated with electric arc furnace controls that precede the present invention is that they rely on oversimplified analyses that do not yield estimations of the slag characteristics to facilitate adequate control of the slag process conditions.
Another problem associated with electric arc furnace controls that precede the present invention is that require excessively frequent offline testing that is impractical under actual electric arc furnace operating conditions.
An even further problem associated with electric arc furnace controls that precede the present invention is that they do not permit operation of the electric arc furnace at optimal energy efficiencies.
For the foregoing reasons, there has been defined a long felt and unsolved need for a electric arc furnace control that facilitates an inexpensive, non-intrusive method for controlling foamy slag characteristics while at the same time maintaining the operability and efficiency of the furnace.